In plastic packaging or molded packaging for integrated circuits, and in ceramic packaging of integrated circuits, a variety of materials are used. These materials are brought together in a die cavity inside a transfer or injection mold machine, and the package assembly is completed by molding the pieces together. After the molded package is created, the device is shipped to a customer. Traditionally, the integrated circuit had long leads extending from it which were inserted into a customers PC board for installation. Currently, the mounting technology for integrated circuits is moving towards surface mount technologies. In surface mounted systems, the integrated circuit is placed in close contact to the customers board on short gull wing or J type leads. The entire board including the integrated circuits is then processed through a solder reflow machine such as a vapor phase reflow system, or solder reflow oven, or other environments which expose the packaged integrated circuits to high temperatures.
During these high temperature processes, the mechanical tolerances within the packaged IC may be exceeded, resulting in thermal stress induced package failures. Some of these failures include package cracking, delamination at the die--encapsulant interfaces, delamination of die attach material, delamination of the lead frame tape from the die surface, and other defects. These defects are usually caused by the vaporization, under high temperatures, of moisture retained in the packaged IC. These defects can cause significant reductions in device reliability and in some cases can cause total device failure. Because this kind of failure happens at a stage where the part is being mounted in the customers system, the final result is that the board must be reworked and the defective part removed, the board made ready for further processing, and another part put into the board. The device failure may also occur after the board is installed in the end users system. At this stage such failures lead to expensive warranty exchanges, repair and rework.
In the prior art, packaged IC's were inspected destructively to detect such defects using probability and statistical analysis. Randomly sampled devices would be stressed in an test environment in an attempt to induce a delamination or package cracking defect. The devices would then have to be destructively taken apart by cross section and analyzed to see if a delamination defect occurred. Hopefully, no defects would be found, however in that case the test procedure has destroyed a reliable, functional device. If a defect is found, then additional sampling is needed to determine whether the lot of devices is suitable for shipping to the customer or not.
The current destructive sampling test procedures are lengthy in time, and cause the loss of devices that "pass" the test. Therefore nondestructive inspection methods have been developed. Typical inspection systems employ scanning acoustic microscopy (SAM) techniques. In a SAM system, acoustical energy is applied to a target device. The energy used is typically applied by a transducer which transmits in the ultrasound frequency range. A receiver is used to receive the signal reflections from the target device. The amplitude and phase information of the received signal may be converted into a visual display, or otherwise analyzed by computer or microprocessor or digital signal processor. Because certain structures are known to cause certain changes or attenuation in the signal, the amplitude and phase of the received signal may be used to identify certain physical structures in the target device.
It has been found that delamination or package cracking defects can be identified using SAM analysis. Although the molded IC package is opaque to visible light, the package material is transparent to sound energy which is in a range suitable for imaging internal interfaces. A large fraction of the incident sound energy is reflected at interfaces of materials, and especially at defects. The defects will usually include a void where one should not occur, and the void is usually filled with air. This will cause a reflection where one should not otherwise occur in a package without defects.
The acoustic probe in a conventional SAM system may be focused at a sufficiently small spot at the depth of certain features within the IC to resolve certain critical defect geometries and to inspect certain features. In performing reliability analysis, the technique is far superior to destructive testing because the test sample may be used again and again to take a variety of different measurements. Also, the same sample package can be subjected to many stresses and inspected to see when the first defects occur, thereby giving data on mean time between failures and lifetime expectancy for the package.
FIG. 1 depicts a prior art scanning acoustic microscope set up 10 for inspecting IC packages. A tank 11 contains de-ionized water as a transmission medium. Integrated circuit 13 is placed in the tank. A three axis positioning system 15 is used to move the transducer 17 about over the IC in the three directions X, Y and Z. The three axis positioning system 15 includes motor controller software running on computer 19, an external stepper motor driver 18 with microstepping capability, and three stages driven with individual stepper motors. Spike pulser/receiver 21 pulses the transducer 17 and receives the reflected energy, which is amplified and fed to a data acquisition board within the computer 19.
Computer 19 may include software for controlling the motors for the three axis positioning system, software to control and respond to the pulser receiver 21, and software to acquire data from the data acquisition board, perform data reduction, and provide visual display of the data acquired.
In operation, the planar lead frame and the co-planar and almost featureless packaging medium of the integrated circuit represent a favorable environment for inspection using SAM. The transducer 17 is used in the pulse echo or C-scan mode. A focused acoustic probe signal is scanned over the plane of the lead frame within the integrated circuit. The energy is transmitted into the tank 11 and into the package 13 in narrow ultrasonic pulses typically less than two periods in duration with a center frequency of 15-25 Mhz. The pulses are repeated at a relatively low repetition rate of 10-20 kHz to permit echo reception by the single transducer. De-ionized water is used as a transmission medium.
In one conventional system, the echo signal returning from the IC is digitized using analog to digital conversion circuitry at a sampling rate of up to 800 million samples/second. The digitized data is reduced using software performing a data reduction algorithm and the resulting reduced parameters are stored in multiple buffers for use in developing images. Typically, an image representing a 3 cm.times.3 cm frame is scanned in less than 3 minutes. The image is then displayed on the display screen of the computer 19 for the operator or user.
Defects are easily identified using this system. To understand why this is so, a simple description of the mechanism for SAM is required. The prior art SAM system for identifying and locating package defects relies on the phenomenon of reflective signal phase inversion.
When the acoustic energy applied to the package 13 of FIG. 1 is reflected back towards the transducer, the signal phase may invert. This can be detected and displayed. A convenient model for this phenomenon is the model of plane sound waves striking, at a normal incidence, a planar boundary between two fluids. The reflection coefficient, R, is defined as the ratio of the amplitudes of the reflected and incident pressure waves: EQU R=(Z.sub.2 -Z.sub.1)/(Z.sub.2 +Z.sub.1) (Equation 1)
The terms Z.sub.1 and Z.sub.2 are the characteristic acoustic impedances of the materials on the incident and the transmitted side of the material to material boundary, respectively. The characteristic acoustic impedance describes the coupling between the instantaneous pressure and particle velocity, and can be calculated from the equilibrium density, .rho., and plane wave speed, c. Thus for any particular impedance characteristic Z.sub.i, EQU Z.sub.i =.rho..sub.i c.sub.i. (Equation 2)
An examination of Equation 1 shows that the impedance mismatch across a boundary determines both the phase and the amplitude of the reflected pulse. For example, when the characteristic acoustic impedance of the second medium is less than that of the first medium, R is negative and the phase of the reflected pulse is inverted from the incident pulse. Also, the greater the difference between the acoustic impedances at the boundary, the greater the reflectivity of the boundary, that is the value for R will increase, so the reflected pulse will have a greater amplitude.
Typically in a packaged IC device, the acoustic impedance increases across a material interface. Examples are package to die interface, package to leadframe, die to tape interfaces. However, at a defect site, the acoustic impedance of the transmitted side of the boundary is less than that of the incident side, that is the encapsulant. This is a boundary between package material, usually plastic encapsulant or resin, and a void within the defect which is filled with air. Thus the interface is an encapsulant/air interface. Air has a very low relative acoustic impedance. Thus at this boundary, the reflected signal will exhibit a phase inversion. The acoustic receiver and data acquisition board can detect phase inversions in the reflected signal.
Experience with packaged integrated circuits has shown that the package material/silicon interfaces will show a greater intensity difference at delamination defect sites than at delamination defects occurring at package material/lead interfaces. However, if a material is present in the package that has a lower characteristic acoustic impedance Z than the package material, this internal interface will also cause a reflected signal phase inversion. The data analysis software operating in computer 19 will then have to use additional data and perform additional analysis in the temporal signal or the frequency domain signal in order to distinguish these phase inversions from delamination defects.
By performing time-of-fight analysis on the acoustic signal echo, it is possible in most cases to resolve the exact location of an internal interface within the package in three dimensions. Because the known internal interfaces can be exactly located, additional phase inversion interface sites can be classified as package defects. However, for some applications the conventional system of FIG. 1 is insufficient, or inappropriate. For example, some packaged IC's are provided in TSOP packaging, that is Thin Small Outline Packages. These packages are only around 1 mm thick when completely assembled. The normal SAM systems of the prior art cannot resolve small depths sufficiently to distinguish between defect sites and the expected internal package interface surfaces in the TSOP packages. Also, because the interfaces within the package and the defects both cause phase inversions in the reflected signal, several passes and several minutes of data reduction and software processing time is required for each image that is displayed. While the instrument described above is very useful for package development and failure analysis lab work, it is too slow and the data collection and reduction steps too awkward to use in a GO/NO-GO or PASS/FAIL type inspection station in a production environment. In producing IC's, it is desirable to have a time and cost efficient, simple to use, fast test station so that samples can be rapidly analyzed for pass or fail. In this kind of environment, it is not necessary to know exactly the location of a defect, so long as the defect can be generally classified. The operator of the equipment in a production environment is typically quite familiar with the package and so does not need to know exactly where the defect is located within the package, typically knowledge of the package type and the type of defect (e.g. cracking, delamination, die attach failure) will be enough information.
Finally, the conventional SAM system cannot analyze some IC packages now in volume production. In order to provide the SAM inspection of such packages, such as the very thin TSOP packages now in use, the existing equipment will have to be replaced with much more expensive SAM systems.
Accordingly, a need thus exists for a method and an apparatus for providing nondestructive scanning acoustical microscopy inspection for very thin I.C. packages and for pass fail reliability testing. The technique and apparatus should use existing systems and should provide a rapid processing and throughput capability for use in production environments.